Currently my wife has a 1 yr shengen visa for Europe as we have visited Europe 2 times in the last 12 months. 

 

We are going to pop into 'Great' Britain for a few weeks. I googled how to get a visa and wanted to know if this is a good option:

 

https://www.vfsglobal.co.uk/th/en/how-to-apply

 

8 steps to get a uk visa.... 

 

Is there a 3 month time limit (prior to our departure) to which we can apply? or any time limit? 

 

I would sponsor her for this visa. Any advice is appreciated

thanks

Your wife and you want to "pop into the UK" for a few weeks, how long is a few weeks, what are your plans for the trip and what is your nationality?

 

Your wife will need to satisfy the ECO, the Entry Clearance Officer who decides whether to issue the visa, that her proposed visit is genuine, affordable and that, on the balance of probablities, she will leave the UK at the conclusion of her visit.

 

All the details of how to apply can be found by folling your link, she cannot apply earlier than three months before her intended date of entry into the UK.

 

There are plenty of threads regarding the application process, and potental pitfalls, after you've had a read through them feel free to come back if you have a specific questions.